The present invention provides a method for varying the melting points and molecular weights of polyolefins in a process of polymerization using metallocene catalysts. The catalysts used in the present invention are chiral and stereorigid and include a bridge between the cyclopentadienyl groups. It has been discovered that changing the structure and composition of the bridge leads to changes in the melting points and molecular weights of the polymer products. It has also been discovered that addition of substituents to the cyclopentadienyl rings also influence these polymer properties. The present invention also includes the ability to control the melting points of polyolefins, particularly polypropylene, by controlling the number of inversions in the xylene insoluble fraction of the polymer chain.
The present invention relates to the use of metallocene catalysts in the production of polyolefins, particularly polypropylene, and the ability to vary certain properties of the polymer products by varying the structure of the catalyst. In particular, it has been discovered that changes in the structure and composition of a bridge linking two cyclopentadienyl groups in the metallocene catalyst changes the melting points and the molecular weights of the polymer products.
The use of metallocenes as catalysts for the polymerization of ethylene is known in the art. German patent application 2,608,863 discloses a catalyst system for the polymerization of ethylene consisting of bis(cyclopentadienyl)-titanium dialkyl, an aluminum trialkyl and water. German patent application 2,608,933 discloses an ethylene polymerization catalyst system consisting of zirconium metallocenes of the general formula (cyclopentadienyl)n Zr Y4-n, wherein Y represents R1CH2AlR2, CH2CH2AlR2 and CH2CH (AlR2)2 wherein R stands for an alkyl or metallo alkyl, and n is used a number within the range 1-4; and the metallocene catalyst is in combination with an aluminum trialkyl cocatalyst and water.
The use of metallocenes as a catalyst in the copolymerization of ethylene and other alpha-olefins is also known in the art. U.S. Pat. No. 4,542,199 to Kaminsky, et al. discloses a process for the polymerization of olefins and particularly for the preparation of polyethylene and copolymers of polyethylene and other alpha-olefins. The disclosed catalyst system includes a catalyst of the formula (cyclopentadienyl)2MeRHal in which R is a halogen, a cyclopentadienyl or a C1-C6 alkyl radical, Me is a transition metal, in particular zirconium, and Hal is a halogen, in particular chlorine. The catalyst system also includes an aluminoxane having the general formula Al2OR4(Al(R)xe2x80x94O)n for a linear molecule and/or (Al(R)xe2x80x94O)n+2 for a cyclic molecule in which n is a number from 4-20 and R is a methyl or ethyl radical. A similar catalyst system is disclosed in U.S. Pat. No. 4,404,344.
U.S. Pat. No. 4,530,914 discloses a catalyst system for the polymerization of ethylene to polyethylene having a broad molecular weight distribution and especially a bimodal or multimodal molecular weight distribution. The catalyst system is comprised of at least two different metallocenes and an alumoxane. The patent discloses metallocenes that may have a bridge between two cyclopentadienyl rings with the bridge serving to make the rings stereorigid. The bridge is disclosed as being a C1-C4 alkylene radical, a dialkyl germanium or silicon, or an alkyl phosphine or amine radical.
European Patent Application 0185918 discloses a stereorigid, chiral metallocene catalyst for the polymerization of olefins. The bridge between the cyclopentadienyl groups is disclosed as being a linear hydrocarbon with 1-4 carbon atoms or a cyclical hydrocarbon with 3-6 carbon atoms. The application discloses zirconium as the transition metal used in the catalyst, and linear or cyclic alumoxane is used as a co-catalyst. It is disclosed that the system produces a polymer product with a high isotactic index.
It is known by those skilled in the art that polyolefins, and principally polypropylene, may be produced in various forms: isotactic, syndiotactic, atactic and isotactic stereoblock. Isotactic polypropylene contains principally repeating units with identical configurations and only a few erratic, brief inversions in the chain. Isotactic polypropylene may be structurally represented as 
Isotactic polypropylene is capable of forming a highly crystalline polymer with crystalline melting points and other desirable physical properties that are considerably different from the same polymer in an amorphous, or noncrystalline, state.
A syndiotactic polymer contains principally units of alternating configurations and is represented by the structure 
A polymer chain showing no regular order of repeating unit configurations is an atactic polymer. In commercial applications, a certain percentage of atactic polymer is typically produced with the isotactic form. It is highly desirable to control the atactic form at a relatively low level.
A polymer with recurring units of opposite configuration is an isotactic stereoblock polymer and is represented by 
This latter type, the stereoblock polymer, has been successfully produced with metallocene catalysts as described in U.S. Pat. No. 4,522,982.
It may also be possible to produce true block copolymers of isotactic and atactic forms of polyolefins, especially polypropylene.
A system for the production of isotactic polypropylene using a titanium or zirconium metallocene catalyst and an alumoxane cocatalyst is described in xe2x80x9cMechanisms of Stereochemical Control in Propylene Polymerization with Soluble Group 4B Metallocene/Methyalumoxane Catalysts,xe2x80x9d J. Am. Chem. Soc., Vol. 106, pp. 6355-64, 1984. The article shows that chiral catalysts derived from the racemic enantiomers of ethylene-bridged indenyl derivatives form isotactic polypropylene by the conventional structure predicted by an enantiomorphic-site stereochemical control model. The meso achiral form of the ethylene-bridged titanium indenyl diastereomers and the meso achiral zirconocene derivatives, however, produce polypropylene with a purely atactic structure.
Further studies on the effects of the structure of a metallocene catalyst on the polymerization of olefins was reported in xe2x80x9cCatalytic Polymerization of Olefins,xe2x80x9d Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, pp. 271-92, published by Kodansha Ltd., Tokyo, Japan, 1986. In this article, the effects of the chiralities, steric requirements and basicities of ligands attached to soluble titanium and zirconium metallocene catalysts on the polymerization and copolymerization of propylene and ethylene were reviewed. The studies revealed that the polymerization rates and molecular weights of the polymers obtained in the polymerization of ethylene with a zirconocene catalyst vary according to the basicity and steric requirements of the cyclopentadienyl groups. The effects of ligands also contributed to the synthesis of polypropylenes with novel microstructures and high density polyethylenes with narrow and bimodal molecular weight distributions.
The present invention relates to discoveries made as to varying the bridge structure and substituents added to the cyclopentadienyl rings in a metallocene catalyst on the polymerization of propylene and high alpha-olefins. In particular, it was discovered that by varying these components, the physical properties of the polymer may be controlled.
As part of the present invention, it was further discovered that the number of inversions in the xylene insoluble fraction may be varied by changing the components that form the bridge between the cyclopentadienyl rings in a metallocene catalyst. It was also discovered that the addition of various substituents on the cyclopentadienyl rings also varied the number of inversions. Thus, a means for varying the melting point of a polyolefin was discovered. This is a significant discovery, as heretofore it was the commercial practice to vary the melting points of polymer products by co-polymerizing varying amounts of ethylene to produce co-polymers with a range of differing melting points. It is desirable to produce a homopolymer with varying melting points without the use of ethylene. The present invention provides a method for the production of homo-polymers with varying melting points by varying the structure of the metallocene catalyst used in the polymerization.
Similarly, it was discovered that by changing the structure of the metallocene catalyst, polymers are produced with varying molecular weights. Thus, the molecular weight of the polymer product may be varied by changing the catalyst. Accordingly, the present invention provides a method for varying both the melting point and the molecular weight of a polymer product.
The present invention also provides a process for the polymerization of olefins comprising contacting an organoaluminum compound with a metallocene described by the formula:
xe2x80x83Rxe2x80x3(C5Rxe2x80x2m)2 Me Qp
wherein (C5Rxe2x80x2m) is a cyclopentadienyl or substituted cyclopentadienyl ring; Rxe2x80x2 is a hydrogen or a hydrocarbyl radical having from 1-20 carbon atoms, each Rxe2x80x2 may be the same or different; Rxe2x80x3 forms a bridge between the two (C5Rxe2x80x2m) rings and contains a bridge group consisting of an alkylene radical having 1-4 carbon atoms, a silicon hydrocarbyl compound, a germanium hydrocarbyl compound, an alkyl phosphine, an alkyl amine, a boron compound or an aluminum compound, and any of these bridge groups may contain any of these or other hydrocarbyl groups attached to the bridge; Q is a hydrocarbon radical such as an alkyl, aryl, alkenyl, alkylaryl or arylalkyl radical having 1-20 carbon atoms or is a halogen; Me is a group 4b, 5b or 6b metal as positioned in the Periodic Table of Elements; 0xe2x89xa6mxe2x89xa64; and 0xe2x89xa6pxe2x89xa63. An olefin monomer is added to the metallocene catalyst and the organoaluminum compound. After the polymerization has taken place, the polymer product is withdrawn. The process is characterized by the fact that it provides control of the melting point of the polymer product by controlling the number of inversions in the xylene insoluble fraction of the polymer. The number of inversions are effected by the Rxe2x80x3 group and the Rxe2x80x2 group. Thus, the melting point of the polymer product may be varied and controlled by varying the Rxe2x80x3 bridge and/or the Rxe2x80x2 substituents on the cyclopentadienyl rings.
The present invention also provides a method for varying the melting points of polymer products and a method for varying the molecular weights of the polymer products. These methods include the use of the metallocene catalyst described by the above formula. The melting points and molecular weights of the polymer products are varied by changing the Rxe2x80x3 bridge and/or the Rxe2x80x2 substituents on the cyclopentadienyl rings.
The present invention provides a method of controlling the melting point of a polymer by controlling the number of inversions in the chain of the xylene insoluble fraction of the polymers. The number of inversions are controlled in turn by the structure and composition of the catalyst, and the number of inversions and hence the melting point of the polymer product may be controlled and varied by varying the catalyst. In particular, it has been discovered that varying the Rxe2x80x3 bridge between the cyclopentadienyl rings will vary the melting point of the polymer product. Varying the Rxe2x80x2 substituents on the rings will also vary the melting point. In addition, it has been discovered that varying the Rxe2x80x3 bridge and/or the Rxe2x80x2 substitutents in the catalyst will also vary the molecular weights of the polymer products. These beneficial advantages will become more apparent from the following detailed description of the invention and the accompanying examples.
Normally, when propylene, or another alpha-olefin, is polymerized in a catalyst system prepared from a transition metal compound, the polymer comprises a mixture of amorphous atactic and crystalline xylene insoluble fractions which may be extracted using suitable solvents. Transition metal catalysts in the form of metallocenes have been known for some time, but up until just recently, such catalysts could only produce predominantly atactic polymer which is not nearly as useful as the isotactic form. It was discovered that by attaching a bridge between the cyclopentadienyl rings in a metallocene catalyst and by adding one or more substituents on the rings to make the compound both stereorigid and chiral, a high percentage of isotactic polymer could be produced. As described by the present invention, the composition of the bridge and the substituents added to the rings affect the properties of the polymer such as melting points and molecular weights.
The metallocene catalyst as used in the present invention must be chiral and stereorigid. Rigidity is achieved by an interannular bridge. The catalyst may be described by the formula:
Rxe2x80x3(C5Rxe2x80x2m)2 Me Qp
wherein (C5Rxe2x80x2m) is a cyclopentadienyl or substituted cyclopentadienyl ring; Rxe2x80x2 is a hydrogen or a hydrocarbyl radical having from 1-20 carbon atoms, each Rxe2x80x2 may be the same or different; Rxe2x80x3 is the bridge between the two (C5Rxe2x80x2m) rings and is an alkylene radical having 1-4 carbon atoms, a silicon hydrocarbyl compound, a germanium hydrocarbyl compound, an alkyl phosphine, or an alkyl amine; Q is a hydrocarbon radical such as an alkyl, aryl, alkenyl, alkylaryl or arylalkyl radical having 1-20 carbon atoms or is a halogen; Me is a group 4b, 5b or 6b metal as positioned in the Periodic Table of Elements; 0xe2x89xa6mxe2x89xa64; and 0xe2x89xa6pxe2x89xa63.
Exemplary hydrocarbyl radicals are methyl, ethyl, propyl, butyl, amyl, isoamyl, hexyl, isobutyl, heptyl, octyl, nonyl, decyl, cetyl, phenyl, and the like. Exemplary alkylene radicals are methylene, ethylene, propylene and the like. Exemplary halogen atoms include chlorine, bromine and iodine with chlorine being preferred.
The preferred transition metals are titanium, zirconium and hafnium. Q is preferably a halogen and p is preferably 2. Rxe2x80x2 is preferably a phenyl or cyclohexyl group such that (C5Rxe2x80x2m) forms an indenyl radical which may be hydrated. As indicated, other hydrocarbon groups may be added to the cyclopentadienyl rings. The preferred Rxe2x80x3 bridge components are methylene (xe2x80x94CH2xe2x80x94), ethylene (xe2x80x94C2H4xe2x80x94), an alkyl silicon and a cycloalkyl silicon such as cyclopropyl silicon, among others. The present invention is such that the Rxe2x80x3 bridge and the Rxe2x80x2 substituents may be varied among any of those compounds listed in the above formula so as to provide polymer products with different properties.
The metallocene catalysts just described are used in combination with an organoaluminum, compound. Preferably, the organoaluminum compound is an alumoxane represented by the general formula (Rxe2x80x94Alxe2x80x94O) in the cyclic form and R(Rxe2x80x94Alxe2x80x94Oxe2x80x94)nAlR2 in the linear form. In the general formula, R is an alkyl group with 1-5 carbons and n is an integer from 1 to about 20. Most preferably, R is a methyl group. Generally, in the preparation of alumoxanes from, for example, trimethyl aluminum and water, a mixture of the linear and cyclic compounds are obtained.
The alumoxanes can be prepared in various ways. Preferably, they are prepared by contacting water with a solution of trialkyl aluminum, such as, for example, trimethyl aluminum, in a suitable solvent such as benzene. Most preferably, the alumoxane is prepared in the presence of a hydrated copper sulfate as described in U.S. Pat. No. 4,404,344 the disclosure of which is hereby incorporated by reference. This method comprises treating a dilute solution of trimethyl aluminum in, for example, toluene with copper sulfate represented by the general formula CuSO4.5H2O. The reaction is evidenced by the production of methane.
The metallocene catalysts used in the present invention are produced using methods known to those skilled in the art. Typically, the procedures simply comprise the addition of the MeQ groups described above and the Rxe2x80x3 group to a starting compound such as indene or some other substituted dicyclopentadiene.
The polymerization procedures useful in the present invention include any procedures known in the art. An example of a preferred procedure would be that disclosed in co-pending application Ser. No. 009,712, filed Feb. 2, 1987, now U.S. Pat. No. 4,767,735, hereby incorporated by reference which describes a pre-polymerization of the catalyst before introducing the catalyst into a polymerization reaction zone.
In the Examples given below, three different polymerization procedures were utilized. These procedures, designated as A, B and C are described as follows:
A dry two liter stainless steel Zipperclave was utilized as the reaction vessel and was purged with 2 psig of nitrogen. An alumoxane solution was introduced into the reaction vessel using a syringe which was followed by the introduction of the metallocene catalyst solution by a second syringe. Approximately, 1.2 liters of propylene are added at room temperature and then heated to the run temperature in 2-5 minutes was then added to the reaction vessel, and the agitator was set at 1200 rpm. The temperature of the reaction vessel was maintained at the run temperature. After 1 hour of stirring, the agitator was stopped, the propylene was vented, and 500 ml of either heptane or toluene was added using nitrogen pressure. The reactor was stirred for 5 minutes and then the contents were poured into a beaker containing 300 ml of a 50/50 solution of methanol/4N HCl. After stirring for 30 minutes, the organic layer was separated, washed 3 times with distilled water, and poured into an evaporating dish. After evaporating the solvent, the remaining polymer was further dried in a vacuum oven.
The procedure is similar to Procedure A except that 1.0 liter of propylene was first added to the reactor. The alumoxane and catalyst were added to a 75 cc stainless steel sample cylinder and allowed to precontact for several minutes before being flushed to the reactor with 0.2 liters of propylene. The remaining procedures were as described in A.
Into a dry 500 cc stainless steel Zipperclave was added 120 cc of dry toluene and the temperature set at the designated run temperature. The alumoxane solution was syringed into the reactor followed by the addition of the catalyst solution by syringe. About 120 cc of propylene was then added to the reactor using nitrogen pressure. After one hour of agitation and temperature control, the agitator was stopped and the propylene vented. The polymer was then extracted as described in A.
These are just examples of possible polymerization procedures as any known procedure may be used in practicing the present invention.
The polymer product may be analyzed in various ways for differing properties. Particularly pertinent to the present invention are analyses for melting points, molecular weights, and inversions in the chain.
The melting points in the examples below were derived from DSC (Differential Scanning Calorimetry) data as known in the art. The melting points reflected in the tables are not true equilibrium melting points but are DSC peak temperatures. With polypropylene it is not unusual to get an upper and a lower peak temperature, i.e., two peaks, and the data reflects the lower peak temperature. True equilibrium melting points obtained over a period of several hours would be 5-12xc2x0 C. higher than the DSC lower peak melting points. The melting points for polypropylenes are determined by the crystallinity of the xylene insoluble fraction of the polymer. This is shown to be true by running the DSC melting points before and after removal of the xylene solubles or atactic form of the polymer. The results showed only a difference of 1-2xc2x0 C. in the melting points after most of the atactic polymer was removed and isotactic polymer remained. The xylene insoluble fraction of the polymer yields a sharper and more distinct melting point peak.
NMR analysis was used to determine the exact microstructure of the polymer including the mole fraction of inversions in the chain of the xylene insoluble fraction. The NMR data may be actually observed or it may be calculated using statistical models. NMR analysis is used to measure the weight percent of atactic polymer and the number of inversions in the xylene insoluble fraction of the polymer.
The molecular weights of the xylene insoluble fractions of the polymers were calculated using GPC (Gel Permeation Chromatography) analysis. For the examples given below, the analysis was done on a Waters 150 C. instrument with a column of Jordi gel and an ultra high molecular weight mixed bed. The solvent was trichlorobenzene and the operating temperature was 140xc2x0 C. From GPC, Mw, or the weight average molecular weight, and Mn are obtained. Mw divided by Mn is a measurement of the breadth of the molecular weight distribution.
As known in the art, the molecular weight of a polymer is proportional to the rate of propagation of the polymer chain divided by the rate of termination of the chain. A change in the ratio leads to a change in the molecular weights. As described by the present invention, a change in the structure of the catalyst leads to a change in the ratio of the rates of polymerization as well as a change in the melting points of the polymer.
The following Examples illustrate the present invention and its various advantages in more detail. The Examples use various zirconocenes to illustrate the invention but similar results would be expected using titanocene, hafnocenes and other metallocene catalysts. The results are summarized in Table 1.